Light extraction from a semiconductor light-emitting device (LED) is limited due to the large optical refractive index, (n.about.2.2-3.8) of the semiconductor material relative to the surrounding ambient, typically air (n.about.1) or transparent epoxy (n.about.1.5). The amount of extraction depends heavily on the macroscopic geometry of the LED and the three-dimensional emission profile of light generated within the active region or light-emitting layer. Before it can escape, most of the light generated within the device is attenuated by the absorbance in the surrounding materials, e.g. epitaxial layers, confining regions, substrate, die attach materials, and electrical contacts.
Typical devices generate photons at the p-n junction that are emitted into a wide range of directions (nearly isotropic emission). As a result, a large percentage of emitted light rays are incident at the device/ambient interface at angles greater than the critical angle for exiting the semiconductor. These rays are internally reflected and are susceptible to absorption within the device. For a typical GaN-based LED, only.about.11% of photons are incident on the top surface within the critical angle (for transmission into epoxy). The remaining light undergoes at least one internal reflection before escaping the chip.
The internally-reflected light in AlInGaN LEDs is particularly susceptible to absorption by the p-layer contact. These contacts must cover essentially the entire p-n junction emitting area because current cannot spread laterally in the semiconductor layers. Since the conductivity of the p-type epitaxial layers is extremely low, typically &gt;20000 .OMEGA./square, current is confined to directly under the contact metal or to within .about.1 .mu.m of the contact edge.
To allow light to escape, AlInGaN LEDs use p- contacts made with extremely thin metal layers. They are typically between 50 and 500 .ANG. thick and made from AuNi or similar alloys. While these thin, `semi-transparent` layers pass most of the light that strikes them near normal incidence, typically greater than 20% of this light is absorbed.
There are several problems with semi-transparent contacts. First, the contacts absorb a large fraction of the LED light. Although they can pass as much as.about.80% of light at near-normal incidence, they are relatively absorptive at angles greater than the critical angle (for light to escape from the LED). Since most LED light is internally reflected, it encounters the partially absorbing contact many times. For LEDs made on sapphire substrates,.about.70% of the emitted light becomes trapped between the absorptive metal surface and the substrate. Since the contact metal rapidly attenuates the intensity of this light, these semi-transparent metal films can absorb the majority of the emitted light.
A second problem is that because the semi-transparent metal films are extremely thin, on the order of a few hundred angstroms, the films do not completely cover rough semiconductor surfaces. This is particularly disadvantageous since rough surfaces can improve light extraction. Semi-transparent films do not conduct current evenly over rough or uneven surfaces and can become discontinuous. This causes the LED to emit light unevenly, or not at all in portions of the device.
A third problem with thin metals is that they are easily scratched, hence creating a discontinuous conducting surface. This makes them difficult to handle, complicating the LED fabrication process.